The administration of CPAP is common in the treatment of Obstructive Sleep Apnea (OSA) syndrome and Upper Airway Resistance syndrome. CPAP treatment may be considered as one form of assisted ventilation generally. The fundamental disclosure of CPAP is made in the specification of PCT/AU82/00063, published under WO 82/03548. CPAP treatment effectively acts as a pneumatic splint of a patient's upper airway by the provision of a positive air pressure a approximately 10 cm H.sub.2 O, although pressures in the range of approximately 2-20 cm H.sub.2 O are encountered. More sophisticated forms of CPAP, such as bi-level CPAP and autosectting CPAP, are described in U.S. Pat. No. 5,245,995. Common to all forms of CPAP is a nose, mouth or face mask fitted to a patient having connection via a flexible air delivery tube to an air flow generator.
In more advanced forms of CPAP treatment, the measurement of airflow in the air delivery tube is used to detect the average volume breathed by the patient and to determine whether that person is inhaling (inspiring) or exhaling (expiring). Currently this is done using an in-line sensor to measure flow directly, or by measuring the pressure drop across a restriction in the air delivery tube (or alternatively, the pressure drop along the air delivery tube). These methods require the use of additional transducers and, in some cases, additional wiring or tubing to connect the transducer to the appropriate point in the circuit. A determination of respiratory flow determined in this way can lead to the determination of patient volumetric flow, which is a clinically useful metric.
In this specification any reference to a "mask" is to be understood as embracing a nose, mouth or combination nose and mouth (face) mask suitable for the administration of CPAP treatment. Furthermore, a "mask" can include nasal prongs (cannulae) that are inserted into the nares at the entrance to the airway.
Finally, CPAP treatment may occur at a single pressure or multiple pressures, such as in bilevel or autosetting treatments.